Automatic system for selectively substituting spare channels for failed working channels in a multichannel multilink communication system



C v3,1 11,624 VELY SUBSTITUTING SPARE CHANNELS HANNELS IN A MULTICHANNEL16 Sheets-Sheet 1 F. S. FARKAS LECTI Nov. 19, 1963 AUTOMATIC SYSTEM FORSE FOR FAILED WORKING C MULTILINK COMMUNICATION SYSTEM Filed Jan. 4,1960 SUA/Nm? .9N/HOM #III 3,1 11,624 L:IJHANNELS 16 Sheets-Sheet 2 VELYSUBSTITUTING SPARE HANNELS IN A MULTICHANNE F. S. FARKAS LECTI Nov. 19,1963 Filed Jan. 4, 1960 QNX N ...El

s73/v/WH3 s/v/fe/o/u /NVENTOR S. FAR/(AS By ATTORNEY F` S. FARKAS Nov.19, 1963 3,1 11,624 AUTOMATIC SYSTEM EoR sELEcTIvELY SUBSTITUTING SPARECHANNELS FOR FAILED WORKING CHANNELS IN A MULTICHANNEI.. MULTILINKCOMMUNICATION SYSTEM 16 Sheets-Sheet 5 Filed Jan. 4, 1960 /Nl/ENTOR E s.FAR/ms BLCDLWAV ATTORNEY Nov. 19, 1963 F. s. FARKAs 3,111,624

AUTOMATIC SYSTEM FOR SELECTIVELY SUBSTITUTING SPARE CHANNELS FOR FAILEDWORKING CHANNELS IN A MULTICHANNEL MULTILINK COMMUNICATION SYSTEM FiledJan. 4, 1960 16 Sheets-Sheet 4 MoN/TOR I I lll I I I CHANNEL MON/ TORCHANNEL MON/ TOR CHANNEL /VE/VTOR A Es. FAR/As g By-%Q Qmmw 1L ATTORNEYF. s. FARKAS 3,111,624

VELY SUBSTITUTING SPARE C Nov. 19, 1963 HANNELS AUTOMATIC SYSTEM FORSELECTI FOR FAILED WORKING CHANNELS IN A MULTICHANNEL MULTILINKCOMMUNICATION SYSTEM 16 Sheets-Sheet 5 Filed Jan. 4, 1960 m. .Mft

ATTORNEY Nov. 19, 1963 F. s. FARKAs 3,111,624

AUTOMATIC SYSTEM FOR SELECTIVELY SUBSTITUTING SPARE CHANNELS FOR FAILEDWORKING CHANNELS IN A MULTICHANNEL MULTILINK COMMUNICATION SYSTEM FiledJan. 4, 1960 16 Sheets-Sheet 6 Nov. 19, 1963 F. s. FARKAs 3,111,624

AUTOMATIC SYSTEM FOR SELECTIVELY SUBSTITUTING SPARE CHANNELS FOR FAILEDWORKING CHANNELS IN A MULTICHANNEL MULTILINK COMMUNICATION SYSTEM FlledJan. 4, 1960 16 Sheets-Sheet 7 7' SM/TTE/P AND RECE/VE/P SW/'CH INIT/Al/vl/ENTOA7 E S. FA RKAS A 7' TOPNEY F. S. FARKAS Nov. 19, 1963 3,111,624 vELY SUBSTITUTING SPARE c HANNELS AUTOMATIC SYSTEM FOR SELECTIFOR FAILED WORKING CHANNELS IN A MULTICHANNEL MULTILINK COMMUNICATIONSYSTEM 16 Sheets-Sheet 8 Filed Jan. 4, 1950 /Nl/E/VTOR F5. FAR/(AS BV.NMR T F Sm @R @om Il uw Il m.. h h En m h 3m h Nbm1 \1 mm. h h hm h FNB,LR R SB Sm Sm S Sm\ kwou E@ vmw\ l l lsb E m97 ld@ El @d m /Im V En 'Il@Gkoo @a w l ATTORNEY m S m l OK A TR Q Non NA WF 9 M ws 6s e Cl. La s y1m B s MWL m QQ t M W mm N A 6 ym 1 E w GUM www T Y w A S l mok X@ NNNSN USM n Q \.I||I33A um? ALWN mQm .3m \Nsm snow kom CG Em h Fmmm n vSRU. l mmm dwf FDL EW new mm mgm mg. iL HR Q No. O O l mF n 3T l IL 6AN\ NQ 9m 4, Q N59@ Sw 1w om Gw mc 35@ ,A m QQ w J .|.j TL i d I v o .1 NF m .mi m

3,1 11,624 CHANNELS FL S. FARKAS Nov. 19, 19,63

AUTOMATIC SYSTEM FOR SELECTIVELY SUBSTITUTING SPARE FOR FAILED WORKINGCHANNELS IN A MULTICHANNEL MULTILINK COMMUNICATION SYSTEM 16Sheets-Sheet l0 Filed Jan. 4, 1960 F. s. FARKAs 3,111,624 AUTOMATICSYSTEM FOR sELEcTTvELY suBsTTTuTTNG SPARE CHANNELS Nov. 19, 1963 FORFAILED WORKING CHANNELS IN A MULTICHANNEL MULTILINK COMMUNICATION SYSTEM16 Sheets-Sheet ll Filed Jan. 4, 1960 Q www Sw() K V wk /Nl/E N7' 0R ES. FAR/(A5 ay-MAAT? ATTORNEY 16 Sheets-Shea?l 12 F. S. FARKAS MULTILINKCOMMUNICATION SYSTEM FOR FAILED WORKING CHANNELS IN A MULTICHANNEL Nov.19, 1963 AUTOMATIC SYSTEM FOR SELECTIVELY SUBSTITUTING SPARE CHANNELSFiled Jan. 4, 1960 A TTORNEV Nov. 19, 1963 AUTOMATIC SYSTEM F s. FARKAs3,111,624

FOR SELECTIVELY SUBSTITUTING SPARE CHANNELS FOR FAILED WORKING CHANNELSIN A MULTICHANNEL MULTILINK COMMUNICATION SYSTEM Filed Jan. 4, 1960 16Sheets-Sheet 15 A TTORNEV Nov. 19, 1963 Filed Jan. 4, 1960 s. FARKAs3,111,624

F. AUTOMATIC SYSTEM FOR SELECTIVELY SUBSTITUTING SPARE CHANNELS FORFAILED WORKING CHANNELS IN A MULTICHANNEL MULTILINK COMMUNICATION SYSTEM16 Sheets-Sheet 14 CASE CONDITION OF OPERATION oF THE SWITCHING SYSTEMI. WORKING CHANNELS, PROTECTION CHANNELS, AND AUxILIARY CIRCUITFUNCTIONING PROPERLY.

2 WORKING CHANNELS ANO PROTECTION CHANNELS FUNCTIONING PROPERLY.AUxILIARY CIRCUIT MALFUNCTIONS REMOvING ALL STATUS TONES.

3. WORKING CHANNEL I REQUESTS A SwITCH. ALL OTHER CHANNELS FUNCTIONINGPROPERLY.

4. WORKING CHANNEL I SwrrCHED. AUxILIARY cIRcurr MALFUNCTIONS REMOI/INGALL SrATUS TONES.

5 WORKING CHANNEL I Swn-CHEO. AUxILIARY CIRCUIT FUNCnONING PRoPERLvRETURNING ALL STATUS TONEs EXCEPT wORKING CHANNEL I.

6. WORKING CHANNEL I SwITCHED. WORKING CHANNEL s REQUESTS A swITCH.

7 WORKING CHANNELS I ANO e SwITcHEO To PROTECTION CHANNELS 7 AND a,RESPECTIVELY.

I OSS OF ALL STATUS TONES.

8 WORKING CHANNEL I REQUESTS SwITCH RELEASE. WORKING CHANNEL s REMAINSswrrCI-IED TO PROTECTION CHANNEL a.

8o.. PROTECTION CHANNEL 7 STATUS TONE MOMENTARILY REMOVED.

9. WORKING CHANNEL s REQUESTS SwITCH RELEASE.

9u. PROTECTION CHANNEL a STATUS TONE MOMENTARILY REMOVED.

IO. RETURN OF SYSTEM TO CASE I..

H WORKING CHANNELI REQUESTS A SwITCH. PROTECTION CHANNEL 7 NOTPUNCTIONING l PROPERLY. ,a WORKING CHANNEL 2 SwITcHEO TO PROTECrIONCHANNEL 7. WORKING CHANNEL I REQUESTS A SwITCH.

/NI/E/vrof? E S. FAR/(AS A from/Ey NOV. 19, 1963 F* s, FARKAS 3,111,624

AUTOMATIC SYSTEM FOR SELECTIVELY SUBSTITUTING SPARE CHANNELS FOR FAILEDWORKING CHANNELS IN A MULTICHANNL MULTILINK COMMUNICATION SYSTEM FiledJan. 4, 1960 16 Sheets-Sheet 15 Fla/5 AIAA7A8 B Cl C6D E7 E8 Fl F6 GIG6HI H6Jl J6 KI K6 L MIM2M3M6N|N2N3N6 OOIIIIIIIIOOOOOOIIOOOOOOOOOOOIIOOOIIIOOOOOOOOIIOOOOOOOOOOO IOIl IOII I IIOIOIOOIOOOIOOOOOOO|IOOOIIIOOOOOOIOOIOOOOOOOOOOO IOII lOl! I I IOIOIOOOOOIOOOOOOO I l IIOOI l II Il Il IOOOOOIOOIOOOO IIOOOIIIOOOOOOI IOOOOOOOOOOOOO Ol I IllOl l IOIOlI IOOIOIOOOIOOOO OIOIOOIOIIOIOIOI IOOOOOOOIOOOOOOIIIIIIIIOOOOOI IOOIIOOOOOOOO OOIOOOOOIIOOOOOOIIOOOOOOOOOOO00|IIIIIIIOOOOOOIIOOOOOOOOOOO IOOIOIOOIIIOIOIOOIOO/O/OOOOIOOO lOl l lOlIl I IOIOIOOIOOOOIOOIOOO /NVEA/TOR F. S. FAR/(AS A 7' TORNE y Nov. 19,1963 F. s. FARKAS 3,111,624

AUTOMATIC SYSTEM FOR sELECTIvELy sUBsTITUTINC SPARE CHANNELS FOR FAILEDWORKING CHANNELS IN A MULTICHANNEL MULTILINK COMMUNICATION SYSTEM FiledJan. 4, 1960 16 Sheets-Sheet 16 TRANSMITTER SWITCH RECEIVER SWITCH NOSWITCHES MADE. NO SWITCHES MADE.

NO SWITCHES MADE. NO SWITCHES MADE.

WORKING CHANNEL I SWITCHED TO PROTECTION WORKING CHANNELI SWITCHED TOPROTECTION CHANNEL 7. CHANNEL 7.

WORKING CHANNEL I SWITCH HELD BY ACTION WORKING CHANNEL I SWITCH HELD BYACTION OF SWITCH HOLD CIRCUIT. OF SWITCH HOLD CIRCUIT.

WORKING CHANNELI SWITCH HELD BY SWITCH WORKING CHANNEL I SWITCH HELD BYSWITCH REQUEST SIGNAL. REQUEST SIGNAL.

WORKING CHANNEL IISWITCHED TO CHANNEL 7. WORKING CHANNEL I SWITCHED TOCHANNEL 7.

WORKING CHANNEL 6 SWITCHED TO CHANNEL 8. WORKING CHANNEL 6 SWITCHED TOCHANNEL 8.

WORKING CHANNELS l ANDIB SWITCHES HELD WORKING CHANNELS l AND 6 SWITCHESHELD BY ACTION OF SWITCH HOLD CIRCUITS. BY ACTION OF SWITCH HOLDCIRCUITS.

WORKING CHANNELS I AND 6 SWITCHED T0 WORKING CHANNEL 6 SWITCH HELD BYSWITCH CHANNELS 7 AND 8, RESPECTIVELY. CHANNEL I REQUEST SIGNAL WORKINGCHANNEL I SWITCH HELD BY ACTION OFSWTI'CH HOLD CIRCUIT. SWITCH RELEASED.

WORKING CHANNEL 6 SWITCH REMAINS. WORKING CHANNEL 6 SWITCH HELD BYSWITCH WORKING CHANNELI SWITCH RELEASED. REQUEST SIGNAL.

WORKING CHANNEL 6 SWITCH HELD BY ACTION OF SWITCH HOLD CIRCUH: WORKINGCHANNEL 6 SWITCH RELEASED.

WORKING CHANNEL 6 SWITCH RELEASED. NO SWITCHES MADE.

NO SWITCHES MADE. NO SWITCHES MADE.

WORKING CHANNEL I SWITCHED TO PROTECTION WORKING CHANNEL I SWITCHED TOPROTECTION CHANNEL 6. CHANNEL 8.

WORKING CHANNEL 2 SWITCHED TO CHANNEL 7. WORKING CHANNEL 2 SWITCHED TOCHANNEL 7. WORKING CHANNEL I SWITCHED TO CHANNEL 8 WORKING CHANNELISWITCHED TO CHANNEL B.

/NvEA/ro@ E S. FAR/(AS ATTORNEY United States Patent O 3,111,624 AUTMATCSYSTEM FR SELECTTVELY SUB- STETUTENG SPARE CHANNELS FR FARBE) WGCHANNELS iN A MULTCHANNEL MULTLlNK CMMUNHCATIN SYSTEM Francis S. Farkas,Wantagh, NX., assigner to Bell Telephone Laboratories, Incorporated, NewYork, NX., a corporation of New York Filed Ilan. 4, 1960, Ser. No. 34414 Claims. (Cl. 32E-2) This invention relates to automatic switchingsystems for multi-channel, multi-link communication systems and moreparticularly to a flexible system wherein any one of a plurality ofworking facilities of such a system are replaceable by any one of aplurality of spare facilities in the event of transmission failure of aworking facility in one or more links of the multi-link system.

`In multi-link communication systems involving terminal stations andseveral repeater stations, failure or significant degradation intransmission in any interstation link disables the entire system. Wheresuch systems are employed for the transmission of information in digitalform or for the transmission of television signals or multiplex messagesignals, such a failure assumes serious proportions `and the provisionof spare facilities becomes economically justified.

A typical radio relay system having one spare facility and devices forautomatically substituting this spare facility for any one of aplurality of working facilities is disclosed in United States Patent2,733,296, granted J. B. Maggio on January 31, 1956. The system thereindescribed contemplates the transmission of tones over the workingfacility for the monitoring thereof to determine the necessity of yasubstitution. When information, such as telephone switching signals,teletypewriter signals or computer data, is transmitted in digital formover the wor-king channels, tones of the type used in the system of theabove-identified patent cannot be used because of the possibility ofinterference.

When there are a number of Working channels and each channel is carryingtelevision signals or multiplex message signals, it is desirable toprovide protection of greater reliability than previously provided. Oneway of providing this greater reliability and protection is to use aspare channel with each individual working channel and means forautomatically substituting the spare channel for its working channelupon failure thereof.

In -a multi-channel system, the provision of a spare channel for eachworking channel (commonly called a one-for-one system) becomes verydiicult because of the large number of components required at theterminal stations yand the fact that each spare channel requires afrequency band as broad as tha-t of the working channel which furthercongests the frequency spectrum at the radio frequencies.

Therefore, it is desirable to provide an automatic switching system thatprovides a plurality of spare facilities, any of which can besubstituted for `any one of a plurality of working channels. Such asystem will therefore require fewer spare facilities to protect a largenumber of working facilities than a system where each working facilityhas its own spare facility.

A system having a plurality of spare facilities capable of beingsubstituted for any one of the working facilities will givesubstantially as reliable a service as will a onefor-one system ingeneral and in some cases will give better reliability and service. Forexample, if a spare facility vfails after substitution, another sparemay be substituted therefor. While such a system is of advantage ingeneral in any multi-link communication system, it is of particularladvantage when used in conjunction with lCe microwave radio rel-aysystems of the type employed for transcontinental transmissions ofmultiplex digital, telephone, -and television signals and specificallywith the type described in the article entitled 6,000-Megacycle-Per-Second Radio Relay System for Broad-Band Long- Haul Service in theBell System, by M. B. McDavitt in the American Institute of ElectricalEngineers Transactions, vol. 76, Part 1, starting at page 715.

In the radio relay system described in the McDavitt article, a one-waysystem is built up of sections each including -a radio transmittingterminal, a radio receiving terminal and several intermediate radiorepeaters. The message waves which fall in the video frequency band areapplied to frequency-modulation equipment at the transmitting yterminalto modulate a 74 meg-acycle-per-second (intermediate-frequency) carrier.This carrier is heterodyned up to a frequency of the order of 6,000megacylesper-second .for microwave radio transmission. At radio repeaterpoints the received wave is reduced to the intermediate frequency,amplified at that frequency and then increased again to a microwaveradio frequency for retransmission. Ultimately, the modulated signal isrecovered at the receiving terminal 'and is demodulated to recover themessage intelligence. interruptions of transmission over such a systemcan be caused by equipment failure or by 4fading which results fromvarying atmospheric conditions.

It is convenient in the `.application of automatic protection switchingto such `a system to isolate for switching purposes, switching sectionsincluding either a terminal station, -a number of intermediate repeaterstations and a so-called main station, which comprises a repeaterstation provided with switching equipment, or two main or terminalstations and the repeater stations therebetween. In either case, theproblem contemplated by the invention involves the compatibility landinterchangeability of a system having only one spare :facility and asystem having a plurality of spare facilities, the recognition atadvantageous points along the transmission route of the failure of aprior section of `a working facility, and lthe substitution of a sparefacility for this section within as short a time as possible.

in accordance with the invention, therefore, an lauto matic switchingsystem is provided for a communication system which includestransmitting and receiving stations, a plurality of working channelslinking these stations through intermediate repeater stations and aplurality of spare or protection channels lalso linking these stations-and an auxiliary circuit including means for effecting a substitutionof a spare channel upon the transmission failure of a working channel.

The automatic switching system includes switches at the transmitter andreceiver terminals for substituting any one of the protection channelsfor any one of the working channels that might fail, channel monitorsbridged across the transmission circuits for originating switch andrestoral requests in accordance with the condition of its particularchannel, and an auxiliary circuit yassociated with the communicationsystem and composed of reporter circuits coupled to the channel monitorsat each location for accepting the signals from the channel monitors'and processing them as to channel and condition for transmission to theswitch control, a transmitter switch control 4at the transmitterterminal for controlling .the switching action of the switches at thetransmitter and receiver terminals, a receiver switch control at thereceiver terminal for accepting signals from the transmitter `switchcontrol and effecting the switching action of the receiver switch, andmeans for carrying the information etween the component circuits of theauxiliary circuit.

These and other features and advantages of the invention will appearmore clearly and fully upon con- 3 V sideration of the followingspecification taken in connection with the drawing in which:

FIG. 1 is a block diagram of a flexible switching system according tothe invention illustra` 'g the c1rcu1t -arrangements for a switchingsection including a transmitter terminal, a receiver terminal, 'a radiorepeater and an auxiliary circuit;

FIG. 2 is -a diagram, partially in block form, illustrating in greaterdetail the equipment provided according to the invention :at -atransmitter terminal;

FIG. 3 is a block diagram of the radio repeater of FIG. 1;

FIG. 4 is -a diagram, partially in block form, illustrating in greaterdetail the equipment provided according to the invention ata receiverterminal;

FIGS. 5 through l2, arranged according to FIG. 13, are diagrams, mostlyin block form, illustrating :in greater detail the equipment providedaccording yto the mvention in an auxiliary circuit;

FIG. 13 is a diagram showing the relative positions of FIGS. 2 through12;

FIGS. 14 through 16, larranged according to FIG. 17, comprise 1a form oftruth table showing the signals .existing at particular inputs andoutputs of the circuitry m the transmitter switch control of FIGS. 5, 6,7, 10, 11 `and 12 for different conditions on the transmission channels;and

FIG. 17 is a diagram showing the relative positions of FIGS. 14 through16.

FIG. 1 shows a flexible switching system according to the inventionincluding a transmitter terminal, a receiver terminal, a radio repeaterand an auxiliary circuit. The switching section shown is similiar to theswitching section employed in `the TH radio relay Isystem as describediby Mr. McDavitt in the article referred to above. Only one radiorepeater is shown for simplicity but there may be from one to six ormore such repeaters depending upon the distance between the transmitterand receiver terminals.

The communication system disclosed by McDavitt includes two auxiliarychannels having a narrow bandwidth and a `different intermediatefrequency than the Working channels and protection channels. One ofthese auxiliary radio channels can be used to connect the elements ofthe :auxiliary circuit of the switching section located at thetransmitter terminal, the receiver terminal, and the repeater stations,or these elements can be connected by by wire lines.

The switching section disclosed includes terminal stations and repeaterpoints; but it is understood that the present invention will operate laswitching section including repeater stations and only one terminalstation or a switching section including only one terminal station. Anexample of a switching `section of only one termnal station is thatexisting where a plurality of microwave channels is terminated at areceiver terminal and the signal from these channels is coupled toaplurality of baseband frequency channels at a transmitter terminal. Boththe microwave channel receiver terminal and the baseband channeltransmitter terminal lare located at the same terminal station and larecoupled not only [by working channels but also by protection channels.

The signals on the working channels, of which working channel 1 istypical, enter the switching section at the transmitter terminal. Herethe signal passes through the channel monitor (which monitors its energylevel), and the transmitter switch 2i? to the radio transmitter di). Thesignal entering the radio transmitter 46 is usually at the intermediatefrequency of the system. This signal may then be heterodyned up to themicrowave frequency and radiated from the transmitter termina-l. At therepeater the radio signal is then received by the radio receiver 50 andis changed to the intermediate frequency of the system. It is thenpassed through channel monitor 60 to the radio transmitter 70. In radiotransmitter 70, the signal is heterodyned up to the microwave frequencyand is reradiated. After being radiated lfrom the last repeater in theswitching section, the signal is received by the radio receiver Sii inthe receiver terminal. The signal frequency is reduced to theintermediate frequency and the signal is passed from the radio receiver8G through a channel monitor 9d yto the receiver switch 12d. Thissignal, after passing through receiver switch 12d, is distributed topoints of utilization :or applied :to a subsequent switching section.

The reliability of the system is improved in yaccordance with theinvention by the automatic substitution of either of two protectionchannels for .a failed working channel in a switching section. Whether aWorking channel in this switching section has failed or no-t isdetermined by evaluating the energy level of the signal received at therepeaters and at the receiver terminal. The channel monitors 60 and 90at the-se points, after evaluating the signal, will notify the auxiliarycircuit o-f the condition of the working and protection channels. F orthe protection channel to be monitored by the channel monitors 6i? and90, a signal mus-t be present :on these channels. Therefore, when theseprotection channels are idle (that is, are no-t replacing lworkingchannels), a monitoring signal is `supplied to Vthem by the protectionchannel IF carrier supply 15d. Also under this condition, provision ismade for absorbing the energy fof this monitoring signal -at thereceiver terminal of the switching section. Therefore, protectionchannel load resistors 170 are supplied to absorb the energy on theprotection channels when they are not replacing a working channel.

When the auxiliary circuit receives signals from the channel monitors6i? and 96 informing it of the condition of the chnanels, the circuitwill effect or not effect a switch to a protection channel as thecondition of the working channel reported by the monitors may warrant.This auxiliary circuit is composed of a transmitter switch control 200,a receiver switch control 4130, a converter Sdi?, a separator andconverter 660, a tone generator 700 and reporter circuits d. Of theseelements, channel monitors 6@ and 90 are located at the repeater andreceiver terminal stations, respectively, as are the reporter circuitsassociated with these monitors. The tone generator 7d@ is located at thereceiver terminal and the converter Sti@ and the separator and converter650 are located at the transmitting terminal. These elements can becoupled by either a radio channel or a wire line.

The tone generator 790 at the receiver terminal generates channel statussignals which are normally transmitted through the reporter circuits S00in tandem to the separator and converter 6Go at the transmitterterminal. When a working channel or protection channel is notfunctioning properly, the reporter circuits 31B@ act in accordance witha signal from the channel monitors 60 or 99 to terminate this channelstatus signal at this point for any particular channel and thus notifythe transmitter switch control 26d of a failure in the particularchannel.

The transmitter switch control 2G@ also receives a channel monitorsignal input from the channel monitors 19 in the transmitter terminal.This signal from channel monitor 10 gives an indication as to whetherworking channel signal failure is due to a fault in a previous switchingsection or in the instant switching section. If the fault is locatedwithin the instant switching section, a signal is sent from thetransmitter switch control 2% to the transmitter switch 26 which effectsa bridging switch from the failed working channel to a selectedprotection channel. Also, when it has been determined that the fault iswithin the instant switching section, the transmitter switch control 2d@sends a signal, which is converted from direct current to tone form byconverter 590, to the receiver switch control 401). The receiver switchcontrol 400 acts in response to this signal to produce a signal which issupplied to the receiver switch 1Z0 to cause a transfer from the failedworking channel to the protection channel to which the connection hasbeen made at the transmitter terminal.

For a better understanding of the operation of the switching system inaccordance with applicants invention, typical component parts as shownin operative relationship in FIGS. 2 through 12 when placed as indicatedin FIG. 13 will be considered in detail.

The logic circuitry used by way of example herein is of the conventionaltype as described in Pulse and Digital Circuits, by Millman and Taub,McGraw-Hill, the Design of Switching Circuits, by Keister et al., VanNostrand Company, Inc., and Digital Computer Components and Circuits, byRichards, Van Nostrand. It is to be noted that in all cases herein a lsignal is to be taken to mean some positive voltage with respect toground and a 0 signal is to be taken to mean approximately groundpotential.

CHANNEL MONITORS (FIGS. 3 AND 4) The channel monitors 60 and 9@ arerepresented by the schematic drawing of a channel monitor 99 in FIG. 4and produce a signal output when the condition of the working channel issatisfactory. The channel monitor 96 consists of a coupling loop 91comprising high Q tuned circuits 92 and 93 and low Q tuned circuit 94.The tuned circuit 94 will have a negligible effect on the transmissionof energy from the radio receiver Sil to the receiver switch 12@ whenthe relative Qs of the tuned circuits are so designed. The energy intuned circuit 94 is magnetically coupled to the circuit 95. The signalcoupled into circuit 95 is rectified by diodes 96 and 97 and smoothed bycapacitor 9S. This signal appears as a direct-current voltage acrosscapacitor 98 and as an input to transistor 99 between the base andemitter electrodes thereof. In the emitter circuit of transistor 99there is a bias source 16%) which serves as a reference voltage, abovewhich the input signal must be to cause conduction through transistor99.

When transistor 99 conducts, there is a current flow through theemitter-collector junction thereof to the reporter circuit Sti@ in FIG.9. The current iiow originates with the source 891 in the reportercircuit Sil@ and will have a path through the winding of relay 892associated with the particular channel that the channel monitor ismonitoring. This signal flowing through the winding of relay 802 willhold the arm of this relay in the righthand position, as shown,indicating that the channel is in proper working condition.

REPORTER CIRCUIT (FIGS. 8 AND 9) The reporter circuit S95 consists ofeight similar voltage sources S61, relays 892 and shunt diodes 803 whichare associated respectively with the channel monitors of the severalchannels. The reporter circuit 806 has an input from each channelmonitor 99 and also an input on conductor S64 of eight single-frequencytones, one for each working channel and each protection channel, fromthe tone generator 79). The reporter circuit Sil() also consists ofeight band-pass filters S65, each of which is resonant at the particularfrequency associated with a particular channel. Each of the band-passfilters tiS consists of the series resonant circuit of an inductor 806and a capacitor S97.

When a channel monitor interrupts current flow through the winding ofrelay 892, the reverse current caused by the collapsing magnetic eld ofthe relay winding will flow through diode 893 instead of transistor 99in the channel monitor 99, thereby preventing damage thereto. Also whenthe channel monitor interrupts the current flow through the winding ofrelay 892, indicating the failure of the particular channel associatedwith the channel monitor, the tone associated with the particularchannel that has failed will be dissipated in resistor 808.

TONE GENERATOR (FIG. 9) Two generator 790, which generates the eighttones of different frequencies that are presented to reporter circuitconsists of eight similar oscillators, the six oscillators 701 beingrespectively associated with the six working channels, and oscillators767 and '708, respectively, being associated with protection channels 7and 8. Each oscillator has a band-pass filter 702 connected between itsoutput and the common conductor 7 05, which is connected to conductor864 in the reporter circuit S00. Band-pass filters 702 are respectivelyresonant at the frequencies of the associated oscillators and preventinterferences between the oscillators. Each of the band-pass filters 702consists of the series resonant circuit of an inductor 763 and acapacitor 704.

The tone generator 709 also includes relays 727 and 72S, which areprovided in accordance with the invention to provide a means fornotifying the transmitter switch control 2d@ of the reception of and theaction upon a switch release request by the receiver switch controlfidi). When one of the relays is activated by a signal from the receiverswitch control 400i', the tone associated with a particular protectionchannel will be removed from conductor 765 and will 'be dissipated by aresistor '796.

SEPARATOR AND CONVERTER (FIG. 8)

The eight tones of channels l through 6 and protection channels 7 and 8,which are of different frequencies, will all :be passed lby reportersSli@ and will be present at separator and converter 699 when the workingchannels land protection channels are in working order. The separatorand converter 6%' consists of eight bandpass filters 6151, each of whichis resonant at the frequency of a different one of the eight tones, sothat the tones can be separated to represent their particular working orprotection channel. Each of these band-pass filters consists of a seriesresonant circuit comprising an inductor 692 and a capacitor 693 which isresonant at the frequency of one of the tones. After the tone passesthrough the series resonant circuit it enters detector 604 where it ischanged into a direct-current signal. There will be eight direct-currentsignals leaving the separator and converter '686' and entering thetransmitter switch control 299, which is represented by the circuits ofFIGS. 5, 6, 7,10, 11 and 12.

TRANSMI IER SWITCH CONTROL (FIGS. 5, 6, 7, 10,11AND12) Thedirect-current signals fromconverter 6th) are impressed npon memorycircuits in the transmitter switch control 299 and serve to inform thisswitch control 290 of the condition of the working and protectionchannels. The transmitter switch control 299, therefore, includes amemory circuit 219 (FIGS. 5 and l0) for each of the working andprotection channels. The memory circuits can be nip-flop circuits or anysimilar circuits that will perform the function of retaining theinformation contained in a first signal until ano-ther signal isreceived containing new information. In the analysis of the transmitterswitch control 26%, signals that will be present to indicate certainconditions of the channels will be used to aid in the understanding ofthe representative embodiment disclosed herein.

The complete circuitry in the transmitter switch control 269 is shownfor only two of the working channels. However, it is understood thatworking channels 2 through 5 will have identical circuitry. Thecircuitry in the transmitter switch control 20h' lfor a 4working channelis represented by blocks in the drawing and consists of memory circuit210', input gate 229, working channel voter circuit 239, loss of statusIsignal protection circuit 259, switch release initiator 2.79 (all inFIGS. 5 and 10), switch hold circuit 260' (FIGS. 6 and 11) andtransmitter and receiver switch initiator 329 (FIGS. 7 and l2). Acircuit common to all fworking channels is the common switch releaseinitiator 289 (FIG. l1).

The transmitter switch control Zitlf further comprises circuitry for theprotection channels as represented by blocks in FIG. l comprisingindividual memory circuits 210 and a common protection channel votercircuit 299.

The transmitter switch control 298 will have present eightdirect-current signals from the converter 669 when all working channelsand protection channels are in working order. Memory circuit 210 of eachchannel will have an input of one of these direct-current signals andwill have an appropriate output signal.

Each memory circuit 2li? of protection channels 7 and 8, respectively,will have a l signal output when the protection channels are in workingorder. This output of each memory circuit 2li? in the protectionchannels appears at points A7 and A8, respectively. The signal appearingat points A7 and A will appear as input signals to protection channel-voter circuit 2%.

The in-put signals to the protection channel voter circuit 296 will beapplied to the two input terminals of AND-NOT gate 292. AND-NOT gate 292comprises inhibitors 293 and 294 interconnected to form said gate. r[Theoutput of gate 292 is 'applied to delay circuit 295 and thereafter toinverter 296, which will invert this signal so that its output Will :bethe inverse of the output from gate 292.

The input signal to -the voter circuit 294i appearing at point A7 willalso be applied to OR gates 34H and 302 and inverter 395. The output ofOR gate 30d will be amplified by ampliier 303 and will thereafter appearat point E7 and at the input to the input gates 2.2@ of working channelsl, 2 and 3. The input to inverter 36S will be inverted and thereafterapplied as an input signal to each of the transmitter and receiverswitch initiators 32% of working channels l through 6, respectively.This input Will preclude the substitution of protection channel 7 whenit fails for any of the working channels.

The input signal to the voter circuit 29) appearing at point A8 vwillalso be applied as an input to OR gates 301 and 362 and inverter 3%. Theoutput of OR gate 302 will be amplied by amplifier Sti-f2- and willthereafter appear at point E8 and at the input to the input gates 226'of working channels 4, 5 and 6, respectively. The input to inverter 366,after inversion thereby, will be applied to each switch initiator 329'of working channels l through 6, respectively, to preclude thesubstitution of protection channel 8 upon its failure.

A l signal will 'appear at the output of the associated memory circuit210 of the protection channels when the protection channels are inworking order. This output of each -memory circuit 21u in the protectionchannels appears respectively at points A7 and A8.

'Ihe output of the memory circuit 210 in the Working channel circuitrywhich appears at points A1 through A6 will fbe a 0 when the workin gchannels are in good condition. This output will appear as inputs to theinput -gate 220 and the switch release initiator 271B. The input gate2.20, which is a three-input AND gate, also has an input from theprotection channel voter circuit 299. The third input to gate 226 comesfrom channel monitor i0 in the transmitter terminal of FIG. 2. A lsignal will appear at this input from the channel monitor in eachworking channel as long as the Working channel in the precedingswitching section is functioning properly. If a particular workingchannel in the preceding switching section is not functioning properly,a 0 signal will appear at this input to gate 220 and will prevent thetransmitter switch control 200 from acting upon a switch request in theinstant switching section for that working channel. Input gate 2.2%determines whether a failure is in a preceding switching section,whether the protection channels are functioning properly, and whetherits associated Working channel is requesting a switch. On the basis ofits input signals it will or will not produce an input to Workingchannel voter circuit 230 to effect a switch.

The output of gate 220 appears as an input to the working channel votercircuit 230. The working channel voter circuit 231i)` also receives aninput from the protection channel voter circuit 290. There is a thirdinput to the working channel voter circuit 230 which is in the form of alockout signal appearing in a feedback loop coming from the switch holdcircuit 260. The working channel voter circuit 23) determines whether aone-for-six protection system is available and is to be used or Whethera two-for-six protection system is available and is to be used.

The output of the working channel voter circuit 230 appears as an inputto the switch hold circuit 260. There is also an input to the switchhold circuit 260 from the loss of status signal protection circuit 250',which, in turn, has inputs from memory circuit 210 and protectionchannel voter circuit 299. The loss of status signal protection circuit25@ is an AND-NOT gate in which, if both inputs are a l there wiil be a0 output; if both inputs are a 0 there will also be a O output, but ifthe inputs are different there will be a l output.

The switch hold circuit 266 outputs appear as inputs to the switchrelease initiator 27 G and the transmitter and receiver switch initiator320 and as lockout signal inputs to working channel voter circuit 230'.

The switch hold circuit 26) provides an output signal that precludes therelease of a transmitter bridging switch until a switch release requestis verified by the receiver switch control 469. If a switch has beenmade and there is a subsequent loss `of tone on the protection channels,there will be an input to the switch hold circuit 260` from the loss ofstatus signal protection circuit 250 that will produce an output fromthe switch hold circuit 260 to hold the switch.

It is generally desirable to assign to the two protection channelsfrequencies which are at the extremes of the frequency spectrum beingutilized by the radio relay system. This is true because when a channelfailure is due to fading, the fade usually occurs only at a particularfrequency or in a small range of frequencies and not over the entirespectrum that is being utilized. Therefore, the possibility of bothprotection channels failing at the same time, due to fading, is veryremote.

There are different ways of assigning working channels to a particularprotection channel. By way of illustration, the protection channelsherein have been assigned so that protection channel 7 will serveworking channels 1 through 3 before it will serve working channels 4through 6 and protection channel 8 will serve working channels 4 through6 in preference to working channels l through 3. The circuit whichprovides this preference of one working channel over another or a groupof working channels over another is the transmitter and receiver switchinitiator 320.

Switch initiator 320 has inputs from the working channel voter circuit236 and switch hold circuit 260 of each respective channel. Switchinitiator 320 has output signals for application to the transmitterswitch of FIG. 2 which will cause a switch to be made or released inaccordance with the signal inputs to switch initiator 320. Switchinitiator 32.0 also has outputs which appear as inputs to receiverswitch control 490 after being converted from a direct-current signal toa tone by converter 590.

The transmitter and receiver switch initiators 320 are shown for workingchannels l, 2, 3 and 6. The switch initiators for working channels 2 and3 are included to show the interconnection between Working channels 1, 2and 3 when these Working channels are assigned to protection channel 7in preference to protection channel 8.

The components of switch initiator 329 that provide this assignment of agroup of working channels to one protection channel in preference toanother protection channel will be better understood when consideredwith the detailed description of the operation of the transmitter switch20u, infra. The circuit components for releasing a transmitter switchwill also be described herein with the detailed description of thetransmitter switch 260.

1. IN A COMMUNICATION SYSTEM, A TRANSMITTING AND A RECEIVING STATION, APLURALITY OF INFORMATION CARRYING CHANNELS LINKING SAID STATIONS, APLURALITY OF SPARE CHANNELS ALSO LINKING SAID STATIONS, MEANS AT EACH OFSAID STATIONS FOR SUBSTITUTING A FIRST ONE OF SAID SPARE CHANNELS FOR AFAILED INFORMATION CARRYING CHANNEL, MEANS AT SAID RECEIVING STATION FORDETECTING THE FAILURE OF AN INFORMATION CARRYING CHANNEL, MEANS AT THETRANSMITTING STATION RE-